1. Field of the Invention
This invention relates to an improved lithium-sulfur dioxide electrochemical cell. More particularly, it relates to the use of a quinone imine dye as an electrolyte additive in lithium-sulfur dioxide cells.
2. Description of the Prior Art
A substantial amount of interest has recently been centered on the development of ambient temperature, high energy density, electrochemical cells which are light in weight and capable of providing a higher voltage than conventional cells such as nickel-cadmium and lead-acid systems or alkaline cells having zinc anodes. The high energy density cell systems which are currently of interest typically involve the use of active metals (metals above hydrogen in the electromotive series of elements which are unstable in an aqueous environment) as anodes in combination with nonaqueous electrolytes. As used herein, "nonaqueous" is intended to mean substantially free of water. Lithium has been of particular interest as an active metal for such high energy density cells since it is the most active of the metals in the electromotive series and has the ability in an electrochemical cell to provide the highest performance in watt-hours per kilogram of all known active metals.
In conventional electrochemical cells, cathode depolarizers are used in a form which will permit an intimate and maximum contact with an external electrical circuit, such as a set of wires connecting the electrodes of a cell, while also effecting a physical separation of the cathode depolarizer from the anode. In such cells, the cathode depolarizer is generally an insoluble, finely divided solid which is either admixed with or used as a coating over an inert conducting material, such as nickel, graphite or carbon rod, which serves as a current collector or cathode. The physical separation of the cathode depolarizer for the anode is necessary to prevent a direct chemical reaction between the anode material and the cathode depolarizer which would result in self-discharge of the cell.
Until recently, it was generally believed that a direct physical contact between the cathode depolarizer and the anode could not be permitted within an electrochemical cell. It has been discovered, however, that certain cathode depolarizers do not react chemically to any appreciable extent with active metal anodes at the interface between the anode and the cathode depolarizer. Accordingly, with materials of this type, it is possible to construct an electrochemical cell wherein an active metal anode is in direct contact with the cathode depolarizer. For example, U.S. Pat. No. 3,567,515 issued to Maricle et al. on Mar. 2, 1971, discloses the use of sulfur dioxide as a cathode depolarizer in such a cell in combination with a lithium anode.
Japanese patent specification (Kokai) No. 56/35371, published on Apr. 8, 1981, discloses that a quinone imine dye, such as methylene blue, can be used as a cathode depolarizer in an electrochemical cell which contains a lithium anode. Similarly, Japanese patent specification (Kokai) No. 59/68184, published on Apr. 18, 1984, and Tobishima et al. in Journal of Applied Electrochemistry, Vol. 14, 721 (1984) have disclosed that a quinone imine dye can be used as a cathode depolarizer in an electrochemical cell which contains a lithium anode and an electrolyte which is composed of a solution of the quinone imine dye and lithium perchlorate in propylene carbonate. However, none of these references contains any suggestion that a quinone imine dye could be utilized in an electrochemical cell except as a cathode depolarizer. More specifically, these references fail to suggest that a quinone imine dye could be advantageously utilized as an electrolyte additive in a lithium-sulfur dioxide electrochemical cell wherein the cathode depolarizer is sulfur dioxide.
Lithium-sulfur dioxide cells which are constructed with conventional electrolytes typically demonstrate substantial deviation from the open-circuit voltage during current flow conditions. This undesirable polarization is particularly serious during charge of rechargeable cells of this type and represents a major obstacle to the construction of a satisfactory rechargeable electrochemical cell which comprises a lithium anode and sulfur dioxide as the cathode depolarizer. The prior art fails to disclose any method for the reduction or prevention of this polarization.
Electrolytes comprised of a solution of lithium perchlorate and one or more tetraalkylammonium perchlorate salts in liquid sulfur dioxide are highly satisfactory for use in rechargeable lithium-sulfur dioxide cells. We have found, however, that these solutions are unstable at high lithium perchlorate and tetraalkylammonium perchlorate salt concentrations. For example, a one molar solution of tetrabutylammonium perchlorate in liquid sulfur dioxide which is saturated with lithium perchlorate typically begins to decompose about one hour after preparation. This decomposition is observed as a yellowing of the solution and the gradual formation of a precipitate. Unfortunately, this decomposition limits the utility of such electrolytes in lithium-sulfur dioxide cells.
In view of the high energy density which is achievable with lithium-sulfur dioxide cells, the undesirable possibility exists that an uncontrolled release of this energy can take place. Indeed, violent explosions have been observed upon overcharge of certain lithium-sulfur dioxide cells. For example, solutions of lithium aluminum chloride (LiAlCl.sub.4) and lithium gallium chloride (LiGaCl.sub.4) in liquid sulfur dioxide have been disclosed as electrolytes for lithium-sulfur dioxide cells in British patent specification Nos. 2,083,942 and 2,124,821 and in Belgian Pat. No. 895,143. However, rechargeable lithium cells constructed with such electrolytes have been found to explode violently when subjected to overcharge or severe mechanical shock.